In digital transmission systems, the transmitted digital signal must be reconstructed after it has propagated through the transmission channel. This process is known as signal regeneration and includes timing of the received signal and reconstruction of the signal amplitude. To achieve these results, the signal regeneration apparatus must decide when to sample the received signal and then it must decide whether each sample amplitude is above or below some specified threshold.
In certain digital transmission systems, such as coaxial cables, the transmission medium is well-controlled and only slight signal delay and distortion occurs. Accordingly, the received digital signal can be regenerated with an acceptable error rate using fixed sampling times. In other digital transmission systems, however, such as those utilizing radio links, the signal distortion and delay introduced in the transmission channel are uncontrollable and often unpredictable. Signal regeneration in such systems using fixed sampling times results in an unacceptable error rate for telecommunications applications.
It is well-known that regeneration errors can be reduced if the sampling times coincide with the so-called "signal-eyes" of changing dimensions and positions. Such signal-eyes are defined by the ensemble of all possible signal transitions over the baud interval. Moreover, the regeneration errors can generally be reduced even further by the alignment of the sampling times with a predetermined position within the signal-eyes. This predetermined position is typically at the center of the signal-eyes where the signal-eye opening is the widest.
Various techniques have been used to adjust the sampling times to coincide with a predetermined position within the signal-eyes. For example, in U.S. Pat. No. 3,534,273, issued to L. C. Thomas on Oct. 13, 1970, a recursive technique requiring rather elaborate circuitry is utilized to continually monitor the signal-eye boundaries. Once the boundaries are determined, the sampling times are adjusted to coincide with the center of the signal-eyes. In another adaptive timing technique, such as disclosed in U.S. Pat. No. 4,376,309, issued to G. L. Fenderson et al on Mar. 8, 1983, a sample of the digital signal taken at a primary sampling time is compared to samples taken at secondary sampling times, where the secondary sampling times straddle each primary sampling time. Adjustment of the primary sampling time is initiated if the amplitudes of all the samples relative to the common threshold are not the same. While all of the foregoing techniques provide satisfactory results in their intended applications, more recent system applications require a degree of accuracy and sensitivity not provided by the prior art. Accordingly, the development of adaptive timing circuitry with still greater accuracy and sensitivity would be desirable.